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JXB Advance Access originally published online on June 23, 2006
Journal of Experimental Botany 2006 57(10):2379-2390; doi:10.1093/jxb/erj210
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Published by Oxford University Press [2006] on behalf of the Society for Experimental Biology.

RESEARCH PAPER

Quantitative contributions of blue light and PAR to the photocontrol of plant morphogenesis in Trifolium repens (L.)

Angélique Christophe* {dagger}, Bruno Moulia {ddagger} and Claude Varlet-Grancher

Unité d'Ecophysiologie des Plantes Fourragères, INRA, Lusignan, F-86600, France

*To whom correspondence should be addressed. E-mail: christop{at}ensam.inra.fr

Shade-avoidance is a major adaptive response of plants, and is usually considered to be controlled by phytochromes through the perception of changes in the red:far red light ratio. However, few studies on the effects of blue light (BL) and of light intensity [photosynthetically active radiation (PAR)] on light-grown plants have been conducted, especially concerning changes in PAR at constant BL. The objective here was to quantify the photocontrol of aerial morphogenesis by BL and PAR. Experiments were conducted varying BL and PAR independently, with three BL levels (4, 38, and 83 µmol m–2 s–1) at constant PAR (300 µmol m–2 s–1) and three PAR levels (338, 705, and 163 µmol m–2 s–1) at constant BL (36 µmol m–2 s–1). Effects on morphogenetic processes were analysed as quantitative modulations of ontogenic trends and response curves were produced. White clover (Trifolium repens L.) was used, as it is a typical shade-avoider displaying the whole syndrome of shade-avoidance in a purely vegetative stage. Morphological responses were strongly controlled by both BL and PAR changes, through antagonist effects on leaf appearance rate and additive effects on petiole elongation. All the other responses appeared to be the indirect consequences of changes in the leaf appearance rates. BL acted as a light signal for plant morphogenesis. However, the PAR control probably implicates two distinct mechanisms, such as a trophic effect and a signal. Both PAR and BL actions involved organ-specific differences, which are central in the control of the shade-avoidance responses.

Key words: Blue light, leaf appearance rate, PAR, petiole growth, photomorphogenesis, plasticity, shade-avoidance, Trifolium repens L


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